A method for producing a high-purity 321 austenitic stainless steel

Abstract

This invention relates to a method for preparing high-purity 321 austenitic stainless steel, which relates to the field of metal forging technology, and includes the following steps: (1) Electric arc furnace primary refining: using scrap steel and ferroalloys as raw materials, the temperature is ≥1650℃ after melting in the electric arc furnace; (2) AOD refining: the primary steel is transferred to the AOD furnace for decarburization, desulfurization and alloying; (3) LF furnace titanium alloying and refining: the AOD steel is transferred to the LF furnace, ferrotitanium is added, and the Ti content in the steel is controlled; argon soft blowing is carried out at the same time; (4) Vacuum degassing (VD): after LF refining, the steel is sent to the VD furnace and kept under a vacuum of ≤67Pa; (5) Ingot casting: the pouring method or protective casting is adopted, the inner wall of the ingot mold is coated with special protective slag, and the ingot is naturally cooled in the mold for ≥4 hours after casting; (6) Hot annealing: the ingot is immediately sent to the heating furnace after demolding and cooled to below 500℃ with the furnace before being taken out. After production using an electric furnace + AOD + LF + VD, the inclusions in this invention have reached a level where only fine B / D inclusions remain, and the rating level is 0.5.

Description

Technical Field

[0001] This invention relates to the field of metal forging technology, and specifically to a method for preparing high-purity 321 austenitic stainless steel. Background Technology

[0002] Conventional 321 stainless steel (UNS S32100), corresponding to standard AMS5645T, typically has the following composition: C≤0.08%, Cr17.00-19.00%, Ni8.00-12.00%, Ti≥5×(C+N)≤0.70%. It is a titanium-stabilized austenitic stainless steel. 321 austenitic stainless steel is a classic heat-resistant and corrosion-resistant steel grade based on 304 stainless steel, stabilized by the addition of titanium. Its typical composition is 18Cr-10Ni-Ti. Through the preferential combination of Ti with C and N to form TiC and TiN, chromium carbide precipitation at grain boundaries prevents intergranular chromium depletion, thus significantly improving the material's resistance to intergranular corrosion under welding and high-temperature service conditions. Simultaneously, this steel grade possesses excellent high-temperature oxidation resistance, thermal strength, plasticity, and weldability, with a long-term service temperature reaching 816℃ (1500℉). It is widely used in high-end equipment manufacturing fields such as aerospace, nuclear power, petrochemicals, energy equipment, and military industry. Currently, the production of 321 stainless steel both domestically and internationally generally adopts the following technical approach: Smelting: Electric arc furnace (EAF) + argon-oxygen decarburization (AOD) or vacuum refining (VD / LF); Forming: Ingot casting, continuous casting; Subsequent processing: forging, rolling, solution treatment, finishing. Relevant product standards include GB / T 1220, ASTM A276, AMS 5645T, GJB 2294A, EN 10088-3, NB / T 20007.1, etc.

[0003] However, the following problems often exist in actual production: 1. Unstable titanium yield: Titanium is easily oxidized, and the yield fluctuates greatly during the smelting process (40% to 70%), resulting in insufficient effective titanium content in the steel and affecting its resistance to intergranular corrosion.

[0004] 2. Difficulty in controlling inclusions: The addition of titanium easily forms large TiN and Ti(C,N) inclusions, which reduce the fatigue performance and surface quality of the steel.

[0005] 3. Poor internal structure uniformity of steel ingots: Conventionally cast steel ingots have low-magnification defects such as central porosity and segregation, which are prone to cracking during subsequent forging or rolling. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing high-purity 321 austenitic stainless steel that can stably control titanium yield, reduce inclusions, and improve the internal structure of steel ingots.

[0007] The technical solution of the present invention: A method for preparing high-purity 321 austenitic stainless steel includes the following steps: (1) Primary smelting in electric arc furnace: using scrap steel and ferroalloys as raw materials, the temperature after melting in electric arc furnace is ≥1650℃; (2) AOD refining: The primary steel is transferred to the AOD furnace for decarburization, desulfurization and alloying; (3) LF furnace titanium alloying and refining: AOD steel is transferred into the LF furnace, ferrotitanium is added, and the Ti content in the steel is controlled; at the same time, argon soft blowing is carried out to promote the floating of inclusions; (4) Vacuum degassing (VD): After LF refining, the molten steel is sent into the VD furnace and maintained at a vacuum degree ≤67Pa to further dehydrogenate, deoxidize and remove inclusions; (5) Ingot casting: The ingot is cast by pouring or protective casting. The inner wall of the ingot mold is coated with special protective slag. After casting, the ingot cools naturally in the mold. (6) Hot annealing: The steel ingot is immediately sent into the heating furnace after demolding and cooled to below 500°C before being taken out of the furnace.

[0008] Preferably, in step (1), the steel content C is controlled to be ≥0.30% and P ≤0.030%.

[0009] Preferably, in step (2) the decarbonization period, oxygen and argon are mixed and blown, and the oxygen:argon ratio is gradually increased in the volume ratio of 5:1→3:1→1:1→1:3→1:4, with the final C mass content ≤0.05%.

[0010] Preferably, after the decarburization period in step (2), 14-18 kg / ton of quicklime and 12-18 kg / ton of 98 fluorite are added to create reducing slag, and deep desulfurization is carried out until the S mass content is ≤0.005%; ferrochrome and ferronickel are added to adjust the Cr mass content to 17.00-17.40% and the Ni mass content to 9.00-9.30%.

[0011] Preferably, the Ti content in the ferrotitanium added in step (3) is ≥70%, and the amount of ferrotitanium added is calculated as 0.60%~0.90% of the weight of molten steel, and the Ti content in the steel is controlled to be 0.40%~0.70%.

[0012] Preferably, the argon soft-blowing flow rate in step (3) is 0.2–0.4 Nm. 3 / t·min, refining time ≥30 minutes.

[0013] Preferably, the vacuum degassing time in step (4) is ≥15 minutes.

[0014] Preferably, in step (5), the superheat of the molten steel is controlled between 40 and 60°C, and the pouring speed is 0.5 to 1.0 t / min.

[0015] Preferably, in step (5), the steel ingot is allowed to cool naturally in the mold for ≥4 hours.

[0016] Preferably, step (6) involves keeping the temperature at 850~900℃ for 2~4 hours.

[0017] The chemical name of 321 austenitic stainless steel is 1Cr18Ni9Ti.

[0018] Test methods for 321 austenitic stainless steel: GB / T 222 Permissible deviation of finished chemical composition of steel; GB / T 223 Chemical analysis method for steel and alloys; GB / T10561 Test for non-metallic inclusions in steel; GB / T 4334-2020 Corrosion of metals and alloys - Test method for intergranular corrosion of stainless steel; GB / T 10561-2005 Determination of non-metallic inclusion content in steel; Zhao Changsheng. Improvement of smelting process for 321 stainless steel [J]. Special Steel, 2018, 39(3):15-18.

[0019] The simple process flow of this invention is: electric furnace + AOD + LF + VD → forging → solution treatment.

[0020] The beneficial effects of this invention are: Stable titanium yield: By adding ferrotitanium in a reducing atmosphere in an LF furnace and then performing VD vacuum treatment, the titanium yield can be stably controlled at 75% to 85%, effectively avoiding the oxidation and burning loss of titanium, ensuring that the effective titanium content in the steel is ≥0.40%, and significantly improving the resistance to intergranular corrosion.

[0021] Inclusions are significantly reduced: the combination of long-term soft blowing in the LF furnace and VD vacuum treatment results in a total oxygen content of ≤15ppm in the steel, and the TiN inclusions are small (≤15μm) and diffusely distributed. The inclusions of types A, B, C and D are all ≤1.0 grade, which meets the requirements of high-end applications. Detailed Implementation

[0022] A method for preparing high-purity 321 austenitic stainless steel includes the following steps: (1) Primary smelting in electric arc furnace: using high-quality scrap steel and ferroalloys as raw materials, after melting in electric arc furnace, the steel output is controlled to have C≥0.30%, P≤0.030%, and temperature≥1650℃; (2) AOD refining: The primary steel is transferred to the AOD furnace for decarburization, desulfurization and alloying. During the decarburization period, oxygen and argon are mixed and blown, with the oxygen:argon ratio gradually increasing from 5:1 to 3:1 to 1:1 to 1:3 to 1:4 by volume, until C ≤ 0.05% is reached. After the decarburization period, quicklime and fluorite are added to form reducing slag, and sulfur is further removed to ≤ 0.005%. Ferrochrome and ferronickel are added to adjust Cr and Ni to the target range. (3) LF furnace titanium alloying and refining: Transfer AOD molten steel to the LF furnace and add ferrotitanium (Ti content ≥70%). The amount of ferrotitanium added is calculated based on 0.60% to 0.90% of the molten steel weight, and the Ti content in the steel is controlled to be 0.40% to 0.70%. At the same time, argon gas soft blowing is carried out (flow rate 0.2 to 0.4 Nm). 3 / t·min), refining time ≥30 minutes, to promote the floating of inclusions; (4) Vacuum degassing (VD): After LF refining, the molten steel is sent into the VD furnace and kept at a vacuum of ≤67Pa for ≥15 minutes to further dehydrogenate, deoxidize and remove inclusions; (5) Ingot casting: The pouring method or protective casting is adopted. The superheat of the molten steel is controlled at 40-60℃, the pouring speed is 0.5-1.0t / min, the inner wall of the ingot mold is coated with special protective slag, and the ingot is naturally cooled in the mold for ≥4 hours after pouring. (6) Hot annealing: The steel ingot is immediately sent into the heating furnace after demolding and held at 850-900℃ for 2-4 hours. It is then cooled in the furnace to below 500℃ and taken out. Its chemical composition by mass percentage is shown in Table 1: Table 1

[0023] Note: Balance is Fe and unavoidable impurities.

[0024] The size of TiN inclusions in the steel ingot is ≤15μm, the coarse and fine series of non-metallic inclusions of types A, B, C, and D are all ≤1.0 grade, and the low magnification structure of the steel ingot (generally loose, centrally loose, ingot shape segregation) is ≤1.0 grade.

[0025] Chemical names and sources of each component: Ferrochromium: FeCr65, C0.10, Cr≥65%, C≤0.10%; Nickel-iron: FeNi50, Ni 48%~52%; Ferrotitanium: FeTi70, Ti 68%~72%, Al≤3%, Si≤0.5%; Ferrosilicon, ferromanganese, lime, and fluorite are common auxiliary materials.

[0026] Functions of each component: C: Increases strength, but too high a value will reduce corrosion resistance; it should be controlled between 0.04% and 0.08%. Ti combines with C to form TiC, which prevents the precipitation of Cr carbides and improves resistance to intergranular corrosion. However, excessive amounts will form TiN inclusions, so the content should be controlled between 0.40% and 0.70%. Cr and Ni: ensure the austenitic matrix and basic corrosion resistance; P, O, H: Harmful elements; the lower the better.

[0027] Example 1

[0028] Specialized equipment: electric arc furnace (≥20t), AOD furnace (≥30t), LF furnace (≥30t), VD tank (≥30t), casting ingot mold and insulation cover.

[0029] A method for preparing high-purity 321 austenitic stainless steel includes the following steps: Step 1 (Electric Arc Furnace): Charge 25 tons of scrap steel and ferroalloy, melt and then blow oxygen to aid melting at a temperature of 1680℃. The resulting steel has a P content of 0.018% and a C content of 0.35%.

[0030] Step 2 (AOD): The molten steel is transferred to a 30-ton AOD furnace. During the blowing period, the oxygen:argon ratio is gradually increased according to the volume ratio of 5:1 → 3:1 → 1:1 → 1:3 → 1:4, with the final C=0.04% and the temperature at 1700℃. After the decarburization period, 400kg of quicklime and 400kg of 98 fluorite are added, and Ar is blown and stirred for 15 minutes, with the final S=0.002%. According to actual production, ferrochrome is added to adjust Cr to 17.20% and ferronickel to adjust Ni to 9.15%.

[0031] Step 3 (LF): Transfer the molten steel to the LF furnace, add 180 kg of ferrotitanium (70% Ti), control the Ti content in the steel to 55%, raise the temperature to 1620℃, and gently blow argon gas (0.3 Nm). 3 ( / t·min) 35 minutes.

[0032] Step 4 (VD): Molten steel is poured into the VD ladle, vacuum is drawn to 60 Pa, maintained for 18 minutes, and then the vacuum is broken.

[0033] Step 5 (Pouring): Pouring method, molten steel superheated to 50℃, pouring speed 0.8 t / min, steel ingot mold size Φ500×2000mm, cover and keep warm for 4 hours after pouring.

[0034] Step 6 (annealing): Immediately after demolding, place in the furnace and hold at 880℃ for 3 hours. Cool in the furnace to 450℃ and remove from the furnace. Its chemical composition is shown in Table 2.

[0035] Example 2

[0036] Specialized equipment: electric arc furnace (≥20t), AOD furnace (≥30t), LF furnace (≥30t), VD tank (≥30t), casting ingot mold and insulation cover.

[0037] A method for preparing high-purity 321 austenitic stainless steel includes the following steps: Step 1 (Electric Arc Furnace): Charge 25 tons of scrap steel and ferroalloy, melt and then blow oxygen to aid melting at a temperature of 1680℃. The output steel has a P=0.030% and a C=0.35%.

[0038] Step 2 (AOD): The molten steel is transferred to a 30-ton AOD furnace. During the blowing period, the oxygen:argon ratio is gradually increased according to the volume ratio of 5:1 → 3:1 → 1:1 → 1:3 → 1:4, with the final C=0.030% and the temperature at 1700℃. After the decarburization period, 450kg of quicklime and 450kg of fluorite are added, and Ar is blown and stirred for 15 minutes, with the final S=0.002%. According to actual production, ferrochrome is added to adjust Cr to 17.40% and ferronickel is added to adjust Ni to 9.30%.

[0039] Step 3 (LF): Transfer the molten steel to the LF furnace, add 225 kg of ferrotitanium (70% Ti), control the Ti content in the steel to 70%, raise the temperature to 1620℃, and gently blow argon gas (0.4 Nm). 3 ( / t·min) 30 minutes.

[0040] Step 4 (VD): Molten steel is poured into the VD ladle, vacuum is drawn to 60 Pa, maintained for 18 minutes, and then the vacuum is broken.

[0041] Step 5 (Pouring): Pouring method, molten steel superheated to 60℃, pouring speed 0.8 t / min, steel ingot mold size Φ500×2000mm, cover and keep warm for 4 hours after pouring.

[0042] Step 6 (annealing): Immediately after demolding, place the product into the furnace and hold at 900℃ for 2 hours. Then, cool it in the furnace to 450℃ before removing it from the furnace. Its chemical composition is shown in Table 2.

[0043] Example 3

[0044] Specialized equipment: electric arc furnace (≥20t), AOD furnace (≥30t), LF furnace (≥30t), VD tank (≥30t), casting ingot mold and insulation cover.

[0045] A method for preparing high-purity 321 austenitic stainless steel includes the following steps: Step 1 (Electric Arc Furnace): Charge 25 tons of scrap steel and ferroalloy, melt and then blow oxygen to aid melting at a temperature of 1680℃. The tapped steel has a P content of 0.015% and a C content of 0.30%.

[0046] Step 2 (AOD): The molten steel is transferred to a 30-ton AOD furnace. During the blowing period, the oxygen:argon ratio is gradually increased according to the volume ratio of 5:1 → 3:1 → 1:1 → 1:3 → 1:4, with the final C=0.05% and the temperature at 1700℃. After the decarburization period, 350kg of quicklime and 300kg of 98 fluorite are added, and Ar is blown and stirred for 15 minutes, with the final S=0.002%. According to actual production, ferrochrome is added to adjust Cr to 17.00% and ferronickel is added to adjust Ni to 9.00%.

[0047] Step 3 (LF): Transfer the molten steel to the LF furnace, add 150 kg of ferrotitanium (70% Ti), control the Ti content in the steel to 40%, raise the temperature to 1620℃, and gently blow argon gas (0.2 Nm). 3 ( / t·min) 35 minutes.

[0048] Step 4 (VD): Molten steel is poured into the VD ladle, a vacuum is drawn to 60 Pa, maintained for 15 minutes, and then the vacuum is broken.

[0049] Step 5 (Pouring): Argon gas is used for protection during the casting process. The pouring method is used, with the molten steel superheated to 50℃, a pouring speed of 1 t / min, and the ingot mold size Φ500×2000mm. After pouring, the mold is covered and kept warm for 4 hours.

[0050] Step 6 (annealing): Immediately after demolding, place the product into the furnace and hold at 850℃ for 4 hours. Then, cool it in the furnace to 450℃ before removing it from the furnace. Its chemical composition is shown in Table 2.

[0051] Table 2

[0052] Note: Balance is Fe and unavoidable impurities.

[0053] The results of the inclusion experiments in Examples 1-3 are shown in Table 3.

[0054] Table 3 Inclusions

[0055] This invention only achieves grade 0.5 for B-coarse and D-coarse stainless steel; and the test data is stable. The 321 stainless steel of this invention can be used as a high-purity austenitic stainless steel.

[0056] Comparative Example 1 The difference between this comparative example and Example 1 is that the LF furnace omits the argon soft-blowing step; all other steps are the same. Experimental results are shown in Table 4. Table 4

[0057] Comparative Example 2 The difference between this comparative example and Example 1 is that the vacuum degassing (VD) step is omitted; all other steps are the same. Experimental results are shown in Table 5. Table 5

[0058] Increase in Class E carbon and nitrogen compounds (Class D): coarse / fine: 1.5 / 0.5.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing high-purity 321 austenitic stainless steel, characterized in that, Includes the following steps: (1) Primary smelting in electric arc furnace: using scrap steel and ferroalloys as raw materials, the temperature after melting in electric arc furnace is ≥1650℃; (2) AOD refining: The primary steel is transferred to the AOD furnace for decarburization, desulfurization and alloying; (3) LF furnace titanium alloying and refining: AOD molten steel is transferred into the LF furnace, ferrotitanium is added, and the Ti content in the steel is controlled; at the same time, argon soft blowing is carried out; (4) Vacuum degassing (VD): After LF refining, the molten steel is sent into the VD furnace and maintained at a vacuum degree ≤67Pa; (5) Ingot casting: The ingot is cast by pouring or protective casting. The inner wall of the ingot mold is coated with special protective slag. After casting, the ingot cools naturally in the mold. (6) Hot annealing: The steel ingot is immediately sent into the heating furnace after demolding and cooled to below 500°C before being taken out of the furnace.

2. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, Step (1) Control the steel output quality content C≥0.30% and P≤0.030%.

3. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, In step (2), during the decarbonization period, oxygen and argon are mixed and blown together. The ratio of oxygen to argon is gradually increased from 5:1 to 3:1 to 1:1 to 1:3 to 1:4 by volume, and the final C mass content is ≤0.05%.

4. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, After the decarburization period in step (2), add 14-18 kg / ton of quicklime and 12-18 kg / ton of 98 fluorite to make reducing slag, and perform deep desulfurization until the S mass content is ≤0.005%; add ferrochrome and ferronickel to adjust the Cr mass content to 17.00-17.40% and the Ni mass content to 9.00-9.30%.

5. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, The Ti content in the ferrotitanium added in step (3) is ≥70%, and the amount of ferrotitanium added is calculated as 0.60%~0.90% of the weight of molten steel, and the Ti content in the steel is controlled to be 0.40%~0.70%.

6. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, In step (3), the argon soft-blowing flow rate is 0.2–0.4 Nm. 3 / t·min, refining time ≥30 minutes.

7. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, The vacuum degassing time in step (4) is ≥15 minutes.

8. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, In step (5), the superheat of the molten steel is controlled between 40 and 60°C, and the pouring speed is 0.5 to 1.0 t / min.

9. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, In step (5), the steel ingot is allowed to cool naturally in the mold for ≥4 hours.

10. The method for preparing high-purity 321 austenitic stainless steel according to claim 1, characterized in that, Step (6) Keep warm at 850-900℃ for 2-4 hours.

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